Calcium is a chemical element. Its symbol on the periodic table (a list of all the elements) is Ca. Its atomic number is 20. (The atomic number says where Calcium sits in the periodic table.) It has 20 protons and 20 electrons (if is an atom, see ion). The most common isotopes are Ca-40 and Ca-44. Its mass number is about 40.08. Calcium is very important in the human body, It is required for minerals and bones.

Calcium is a soft white-gray metal. it is a solid and is opaque. It is an alkaline earth metal. It melts at a quite high temperature for a reactive metal. It is a little harder than lead. It has two allotropes. It does not conduct electricity as well as copper, but is much lighter in weight.

It reacts with water to produce hydrogen and calcium hydroxide. It reacts with water very fast when it is powdered. When it is in a chunk, it starts reacting slowly because calcium hydroxide makes a coating that does not dissolve on the calcium. If a little acid is added to calcium hydroxide, it dissolves it, making the calcium react very fast. It burns when powdered to make a reddish flame. This makes calcium oxide. It also makes calcium nitride when heated. It can react with halogens to make calcium halides like calcium chloride with chlorine.

Calcium forms chemical compounds in the +2 oxidation state. Calcium compounds are colorless. Most calcium compounds are not toxic. They are needed in the human body, actually. They are unreactive as far as calcium ions go. Calcium oxide was used to make limelights, which have a flame heating calcium oxide and makes it glow very bright.

It is important to know how cells work. Many cells have calcium channels on their surface. These are openings where calcium ions can enter the cell. The cell is told to act and it opens the channels. Once in the cell calcium ions activates many proteins to do specific things. For example, when it goes into muscle cells, it makes them contract (shorten so the muscle pulls.) When it goes into nerve cells, it triggers electrical impulses that send a messages. When it goes into white blood cells it makes them fight germs.

Calcium ions are important to cells, but too many calcium ions can be bad. If a cell gets more calcium ion than it needs it can die. This is why the amount of calcium ion in cells is highly regulated. Conversely, not enough calcium ion is bad. Cells must have the right amount to function properly.

Sometimes cells are unhealthy and need to die in for the body to replace them with new, healthy cells. This keeps the whole organism healthy. Cells know when they should die and can trigger reactions to end their life cycles in many ways. When this happens it is called apoptosis, also known as a 'programmed cell death' (planned cell death.) One way cells accomplish apoptosis by taking in toxic levels of calcium ions.

The body controls calcium levels with many hormones. Calcitonin, Parathyroid hormone (acronym PTH), and Vitamin D are the most important hormones in calcium metabolism. (Vitamin D is a hormone but it is called a vitamin.)

Calcium compounds are also important in chemistry. It is important for making things. It is a part of cement which is needed to make concrete (a hard substance that many buildings are made from.)

Calcium is part of calcium oxide. Calcium oxide is used to make paper, pottery, food, and to purify water (make it good to drink.) Calcium carbonate is used as a calcium supplement. Calcium permanganate can be used as a rocket propellant.

Calcium is toxic as an element. It reacts with water and makes a strong base, calcium hydroxide. Calcium compounds are not toxic unless the anion is toxic. Calcium permanganate is only toxic because of the permanganate, not the calcium.

Calcium must be stored in underground bunkers, to avoid reduce the risk of the helvetica scenario.